1. Field of the Invention
The present invention relates to a car navigation system and a program thereof, and a method for updating map data used in the car navigation system.
2. Description of the Related Art
In recent years, navigation systems for vehicles driving on a road (hereinafter referred to as “car navigation system”) have been used in all over the world.
A car navigation system locates a self-location based on map data stored in a Hard Disk Drive (HDD) etc. and latitude/longitude data acquired via radio waves sent from a Global Positioning System (GPS) satellite, and displays a map of the located self-location and its vicinity on a display.
In addition, a car navigation system performs a route search from the self-location to a destination by locating the destination on the map data according to the user's operation, then displays the searched route on the map on the display, and provides an image and audio guidance for route guidance when the vehicle drives along the searched route.
Map data for a car navigation system is stored in a format so called, KIWI format, A format, and I format and the like. Conventionally, it is stored in portable storage media such as a Compact Disk-Read Only Memory (CD-ROM) Digital Versatile Disk-Read Only Memory (DVD-ROM), and distributed to an end user through a car navigation system vendor.
When the new map data is provided with portable media, the end user of a car navigation system updates the map data by replacing old map data in a hard disk drive with the new map data in the portable media.
However, considering the fact that data to be updated is usually at most a few percents of the total map data, it is not an efficient way to update map data via portable media. In addition, in such a method using portable media, it is difficult to keep up with the changes in a road map caused by daily road constructions and repairs. Furthermore, provision of such new map data becomes available usually at most once a year, and as a result, considerable number of places include deviation between map data in a car navigation system and the actual road conditions.
In light of these situations recently, such services started that generate map difference data between an old map and a new map, and distribute the map difference data via communications and broadcasting. In such cases, since no portable media is used, troublesome tasks in vendors and users can be reduced. As a result, the updating frequency of map data can be increased from once a year to once in every two or three months.
However, in the case of distributing map difference data, since communications and broadcasting are used as a distribution means, it takes long time for the distribution and update despite using difference map data to reduce the data size. At present, it takes about 30 minutes for updating map data for Tokyo only.
For example, JP1999-95657 discloses a method for updating map data in which, map data before update is temporarily saved when updating map data, and operations of the car navigation system is continued using the map data before update. When updating entire map data is completed then the car navigation system switches to the operation using the new map data.
In addition, JP2003-315056 discloses an example of a car navigation system in which, map data is formed hierarchically and map data that includes the current vehicle location is updated to reduce the map distribution time and cost. The car navigation system temporarily stores both new and old map data with regard to the updating map data, and performs route guidance using the new map data when self-location is in the updating area, and using the map data before update when the self-location is outside the updating area.
According to the method for updating map data disclosed in JP1999-95657, a car navigation system can continue normal operation even when updating map data is in progress. However a problem is still remained that it takes long time before new map data becomes usable. In addition, since it uses map data before update when updating map data is in progress, there is a possibility of displaying guidance information that does not match the real situation.
According to the car navigation system disclosed in JP2003-315056, there is a possibility of mismatches between data near the border of updated map data and the adjacent map data that is not updated yet, causing inconsistency between displayed map data and route guidance data.
As stated above, in conventional techniques, it is difficult to satisfy both of, making the updated map data to be usable rapidly, and performing a route search and displaying guidance data without inconsistency using map data before update and map data after update.